Why can't I use GPU rendering in Blender?
Blender Cycles relies on GPU compute languages like CUDA or more recently OpenCL. CUDA is exclusive to Nvidia and while OpenCL is technically cross platform, in practice AMD's implementation is the only one that provides a usable speedup for Cycles. 

From the Blender wiki:

https://wiki.blender.org/index.php/OpenCL

... Intel GPU’s are getting more powerful but are not a good target yet and CPU based opencl provides little or no benefit over CPU based cycles.